Paper jams are a common occurrence in sheet-fed xerographic copiers, facsimile machines, printing presses, and the like. In addition to mechanical disruption and potential damage to the apparatus, a paper jam often causes the image-transfer medium, e.g., xerographic toner or printing ink, to be applied to a roller or support element where it adversely affects subsequent copying or printing.
It is, therefore, advantageous to develop a system that will detect incipient and potential misfeeding of sheet material before an actual mechanical jam has occurred, and which will cooperate with other elements of the process to provide a signal indicating such a problem or actually inhibiting further operation.
One solution to this problem is described in U.S. Pat. No. 3,684,890, which covers a photocell misfeed detector system described in operation with a xerographic copying machine. In this system, a photolamp directs two beams of light through fiber optic means to reflect off the surface supporting the sheet material and off the gripper member gripping the paper being fed thereon. If a sheet of material is properly fed to the surface supporting the sheet material, the beam of light striking the gripper finger and the beam striking the paper produce unequal reflective values to thereby indicate a sheet is properly clamped. If the two beams of light are reflected in a substantially equal or balanced condition, a misfeed or unclamped sheet is indicated and a control signal is produced to effect a discontinuance of the operation of the particular machine utilizing the sheet.
It is possible, however, for this prior art system to indicate a properly fed sheet when, in fact, the sheet may be skewed, i.e., not in proper registration on the drum, or not actually gripped by the plurality of clamps required to properly hold the sheet on the drum. For example, the sheet may be skewed or adjacent to the clamp but not under the clamp. A further disadvantage of this system is that it requires three photocells per detection mechanism (or per clamp), two for paper sensing in the area of the clamp and one to determine whether the light source is operating.